If terms of an arithmetic sequence are added together an arithmetic series is formed.
2 + 4 + 6 + 8 is a finite arithmetic series
2 + 4 + 6 + 8 + ... is an infinte arithmetic series
To find the sum of the first n terms of an arithmetic sequence use the formula:
Sum of first n terms of arithmetic sequence d = common difference 
OR
An equivalent formula involving the last term, l
Sum of first n terms of arithmetic sequence 
Example 1

What is the sum of the first 15 terms of the arithmetic sequence: 3, 6, 9, 12, ... 


Example 2

The first term of an arithmetic sequence is 5 and the last term is 250. The sum of this series is 1020. How many terms does it have? 


Example 3

An athlete does 20 pressups on the first day of a training routine. On the second day she does 24 pressups and on the third day 28 pressups. If she follows this pattern for 30 days, how many pressup will she have done altogether? 
